So I've been doing my fair share of research on all sorts of spells. I now know that it's not good to perform a love spell on anyone because it interferes with their free will and will only fire back on you. I have no interest in casting any love spells but I was just wondering, what would happen when you cast the spell on one who already loves you?

depends on what spell you would cast on the person who loves you? to stop them from loving you, yes, you're messing with their free will and bad things can happen. it's also not permanent.

a love binding spell, similar to a handfasting? [Wiccan Wedding] that should be fine as two parties typically enter into it willingly. though i have come across my fair share of binding spells to prevent cheating, and to keep the person with you out of fear he/she wishes to leave you.

go with the basic rule that if the two parties willingly enter into the spell, it's fine. if one wishes to cast over the other as a result of insecurity, that's wrong and can backfire.